This invention relates to methods and apparatus for detecting electrostatic charges on objects such as semiconductor substrates.
Semiconductor devices are notoriously sensitive to electrostatic charge that may build up on wafers during processing, and on people and machines that manipulate the wafers during processing. Excessive charge accumulated on a wafer may cause electrostatic discharge through thin layers of insulation, or electro-erode minute conductive patterns on the surface of a wafer with concomitant destruction of circuits formed on the semiconductor wafer. As such circuits progressively increase in complexity and density on semiconductor wafers, the propensity to be destroyed by lower levels of accumulated static charge also increases. Disk drive and flat panel displays and processing masks are comparably sensitive. Additionally, electrostatic charge on a substrate promotes electrostatic attraction of particulate contaminates that adhere to the surface and disrupt the uniformity of subsequent surface deposition processes. Also, discharge of accumulated electrostatic charges on substrates may generate electromagnetic interference signals that can disrupt proper operations of computer-controlled handling and monitoring equipment. Therefore, significant effort must be expended to prevent static charge build-up on isolated semiconductor wafers and other substrates during processing into finished electronic components.
In accordance with one embodiment of the present invention, an accumulation or stacking of semiconductor wafers or masks into a holding bin or cassette can be monitored for changes in accumulated charge of the stack during insertion or removal of individual wafers within the stack. Standard cassettes for handling multiple wafers through automated loading operations relative to semiconductor processing equipment are modified to provide conductive surfaces that can be connected to charge-measuring meters. In this way, detection of a wafer that includes an excessive accumulation of static charge can be identified and specifically processed to remove the charge and mitigate the prospects of damage to the affected wafer as well as damage to other wafers in the stack. Other wafers or substrates for such devices as flat-panel LCD displays and blanks for hard-disc memory products may be similarly monitored and treated.